We have a great deal of demand for the ability to describe data formats that include layered transformations - that is, regions within the data that need to be algorithmically transformed before parsing and after unparsing.
The IETF data formats make extensive use of line folding, base64, etc. Many formats allow compressed payloads. All these are examples of "layering" where a region of the data stream can be identified, algorithmically transformed, and then subsequently further parsed. (The inverse process for unparsing obviously.)